Acceleration and Gear Choice

[Floriante]

Lets say you are driving in a flat road with the speed of 45mph. You wanna accelerate to 55 mph. Here are two different situtations.

A) You are already in 5th gear while cruising in 45. You accelerate to 55mph in 5th gear
B) You are already in 4th gear while cruising OR you are in 5th gear but you shift to 4th gear and accelerate to 55mph.

In which situtation you consume less fuel? After all when in 5th gear, you accelerate in lower rpm. So?

[California98Civic]

Originally Posted by Floriante

Lets say you are driving in a flat road with the speed of 45mph. You wanna accelerate to 55 mph. Here are two different situtations.

A) You are already in 5th gear while cruising in 45. You accelerate to 55mph in 5th gear
B) You are already in 4th gear while cruising OR you are in 5th gear but you shift to 4th gear and accelerate to 55mph.

In which situtation you consume less fuel? After all when in 5th gear, you accelerate in lower rpm. So?

Depends on the car and how you accelerate relative to the ideal for your car, but in my 98 Civic DX Coupe... I would chose fifth, which is at about 2000 rpm at 45mph, and at 75-80% engine load that would be better for mpg.

[Arragonis]

I agree with 98 but I'm interested in specific answers. Lets say you were driving a small, er, Toyota with 998cc and 68 hp with 22mph/1000 rpm gearing in 5th. Which is better ?

(I know, take this thread to Cuba...)

[Thymeclock]

I'm assuming that this is a manual transmission and you have no need to accelerate quickly, yes? On a flat road, as you described, just leave it in 5th and accelerate gradually.

If you were going up a hill I might recommend downshifting to 4th, otherwise you would probably end up lugging the engine, and stomping on the gas pedal to compensate.

[Arragonis]

Originally Posted by Old Tele man

...acceleration needs HP and that's a function of torque-times-rpms, so you want a gear that positions you just below the engine's peak HP point...and (optimally), reaches its peak HP speed at exactly the same time you reach the desired ROAD speed, so you can shift UP in gear and engine loading and DOWN in engine speed.

...it's not uncommon to routinely accelerate in 4th, skip 5th, cruise in 6th, etc.

So if I wanted to accelerate should I jump from say 5th to 3rd (2000 rpm-ish to 4000 - peak HP is 6000, torque 4000)

EDIT Peak torque 4800...

[dcb]

This is one place where bsfc charts come in handy, they tell you the most efficient operating parameters for your engine. Arag, I would think 4000 is a little high, dunno.

To extract the most power from a given amount of fuel in this example (02 TDI), I want to center my shifts on 1750rpm and have the throttle wide open.


On my saturn I want to center the shifts at about 2500rpm and maintain about 70% throttle.


From the field folks seem to do well with a slightly less aggressive acceleration profile, and of course stop accelerating early to avoid mashing the brakes.

[California98Civic]

dcb,

How did you get these maps? Would I go to a dealer? A mechanic? The internet hunt has never turned up results for me. I rely on PaleMelanesian's extensive experience to identify a sweet spot. I'm confident it is really good... sure would like to see a map. Any tips?

Originally Posted by dcb

This is one place where bsfc charts come in handy, they tell you the most efficient operating parameters for your engine. Arag, I would think 4000 is a little high, dunno.

To extract the most power from a given amount of fuel in this example (02 TDI), I want to center my shifts on 1750rpm and have the throttle wide open.


On my saturn I want to center the shifts at about 2500rpm and maintain about 70% throttle.


From the field folks seem to do well with a slightly less aggressive acceleration profile, and of course stop accelerating early to avoid mashing the brakes.

[Varn]

I run my rpm very low on my diesel vw once it is warmed up. am in 5th gear at 30 mph unless I am on an upgrade.

Vehicle is pre computer so it is seat of the pants. Just keep the engine from making noise. Less noise=less fuel.

[320touring]

Funnily enough, my 318iS has an MPG meter under the RPM counter..

Doing exactly as you describe, back to back seems to suggest that leaving it in 5th, and aiming to have the engine consumption at 20mpg gives the best accel/consumtion/elapsed time combo.

[Varn]

a computer could improve the efficiency but getting the car in top gear early is a good thing for economy, I do it for all my vehicles including my 2 ton econoline.

Originally Posted by 320touring

Funnily enough, my 318iS has an MPG meter under the RPM counter..

Doing exactly as you describe, back to back seems to suggest that leaving it in 5th, and aiming to have the engine consumption at 20mpg gives the best accel/consumtion/elapsed time combo.